(1) Field of the Invention
The present invention relates to a mobile station and a soft handoff method, and more particularly, to a mobile station for performing communication in different cell environments and a soft handoff method for performing soft handoffs in different cell environments.
(2) Description of the Related Art
In recent years, mobile communication technique is switching from analog mobile communication to digital mobile communication with a view to enhancing the speech transmission capacity. Access procedures adopted in the digital mobile communication include TDMA (Time Division Multiple Access) in which users are allocated respective time slots sharing the same frequency, and CDMA (Code Division Multiple Access) utilizing spread spectrum communication technique. Currently, in view of further expansion of the speech transmission capacity, CDMA which permits a greater increase in the number of subscribers than TDMA is attracting attention as a key technology for the next-generation mobile communication.
Meanwhile, in the case where a mobile station moves beyond a cell during communication, it is necessary that the base station with which the mobile station is communicating be switched to the base station in the cell to which the mobile station is moving, in order to ensure continuous communication. This switching control is called handoff.
In CDMA in general, since adjacent cells can use the same carrier frequency, it is unnecessary to synchronize carrier frequency at the time of handoff, and only spreading code needs to be synchronized. Accordingly, near the boundary between cells, signals from a plurality of base stations can be received at the same time, thus making it possible to perform the function called soft handoff that cannot be achieved with mobile stations connected in multiple access mode according to TDMA. When adjacent base stations are to be switched as a mobile station moves, the soft handoff permits the switching to be carried out instantaneously without being noticed by the user. The soft handoff technique has great importance to data communication using portable telephones, rather than the communication of voice information only.
In the case of communication in multiple access mode according to CDMA, however, the carrier frequency of one mobile station is nothing but interference for other mobile stations, and the degree of the interference depends on the transmission power of the carrier frequency. Especially, in a cell environment called umbrella in which a macrocell and a microcell overlap each other, or in a cell environment such as a multicell environment, the interference wave can occasionally become too strong as compared with the target radio wave. This occurs, for example, in the case where a target base station is relatively distant from a mobile station while another base station transmitting the carrier frequency is situated very near the mobile station. Such an undesired wave from a near location causes remarkable level interference. Thus, in the cell environments mentioned above, different carrier frequencies must be used.
FIG. 10 is a diagram illustrating how soft handoff is conventionally performed in a cell environment using different carrier frequencies. Normal mode represents a time period in which no soft handoff takes place, and compression mode represents a time period in which soft handoff is performed. In the compression mode during the soft handoff period, received data RX is compressed to 1/2 as compared with data RX received in the normal mode, and using the time saved in this manner, soft handoff is performed through synchronization with a destination-side base station toward which the mobile station is moving. This time division process permits simultaneous communication.
However, the soft handoff mentioned above is susceptible to interference because the spread ratio is 1/2. If the transmission power is doubled to compensate for the drawback, then other mobile stations are subject to interference, entailing a consequent reduction in the speech transmission capacity.
Further, in the case where the reception levels of signals from surrounding base stations are measured by the time division process as described above, a problem arises in that the speech transmission capacity further lowers.